1. Field of the Invention
The present invention relates to an intake air density sensor, and more particularly to an intake air density sensor for an internal combustion engine with an electronically controlled fuel injection system which is required to control the fuel injection correspondingly to the intake air mass flow and therefore required to convert signals obtained from a volumetric flow detector such as the Karman vortices flow meter generally used in detecting the intake air flow into signals exactly corresponding to the intake air mass flows.
2. Description of the Prior Art
When a determined amount of a gas enclosed in a vessel is kept under the same pressure at the same temperature as those of a fluid to be measured, the volume of the gas changes proportionally to variations in pressure and temperature. Accordingly, a density of the fluid to be measured can be determined in a reciprocal by measuring the volume of the gas enclosed in the vessel under the above condition.
In density sensors of prior art according to such a principle, a gas is hermetically sealed in a vessel made of a rubber diaphragm, metal bellows or aneroid barometer vessel which deforms to equalize pressures in and out thereof. The deformation of the vessel is converted into a movement of a slider of a potentiometer to detect the variation in volume of the sealed gas and hence the variation in density of the fluid to be measured. Such density sensors have encountered various problems in elastic restoration of the diaphragm or bellows, resistance to the deformation, frictional resistance of sliding portions, inclination of the slider of the potentiometer or the like, which would cause errors in measurements. Accordingly, it has been difficult to detect in a high sensitivity the delicate variation in density of the fluid, if not impossible, and the devices are very sophisticated in construction and expensive to manufacture.